In recent years, global warming caused by greenhouse effect of carbon dioxide (CO2), which is a combustion product of fossil fuels, has become a concern. With this problem in mind, a method of separating and collecting carbon dioxide from combustion exhaust gas by bring the combustion exhaust gas into contact with an amine-based absorbing solution, and a method of storing collected carbon dioxide without discharging carbon dioxide to the atmosphere have been extensively studied for thermal power plants that use a large amount of fossil fuels.
Specifically, a carbon dioxide recovery system including an absorption tower in which carbon dioxide in the combustion exhaust gas is absorbed into an absorbing solution by bringing the combustion exhaust gas into contact with the absorbing solution, and a regeneration tower in which the carbon dioxide is discharged from the absorbing solution by heating the absorbing solution in which the carbon dioxide has been absorbed is known. The absorbing solution after regeneration in the regeneration tower is again circulated to the absorption tower to be reused.
However, when using the amine-based absorbing solution in the system, an amine component is gradually decomposed by heat and oxygen in the combustion exhaust gas. In addition, when the processed combustion exhaust gas is discharged from the absorption tower, a portion of the amine component is lost to the atmosphere along with the combustion exhaust gas. As a result of degradation and loss, the amine component in the absorbing solution is reduced, and thus the performance of the system is reduced. For this reason, in general, it is necessary to periodically perform replenishment of the amines and replacement of the absorbing solution in the system.
As the amines used for the absorbing solution, liquid amines, which are liquid phase at ordinary temperature (hereinafter, referred to as “liquid amines”), and solid amines, which are solid phase at ordinary temperature (hereinafter, referred to as “solid amines”), can be adopted. In this context, “ordinary temperature” means about 20° C. and may also be referred to as “room temperature” or “standard temperature.”The liquid amines are generally easier to add to the absorbing solution in the system for purposes of replenishing the absorbing solution. In contrast, since it is typically necessary to stir and/or heat for a long period of time to dissolve the solid amines in water, it is more difficult to replenish the solid amines added in the absorbing solution. Furthermore, since replenishment of solid amine components typically requires that water (used to dissolve the solid amines) also be newly added to the system, there is a concern that the amine concentration in the circulating absorbing solution is reduced, and plant performance will be reduced. For this reason, it is preferable to use high concentration amines for purposes of replenishment or to adjust the amine concentration in the circulating absorbing solution by removing excessive water from the system.